AR15-platform firearms comprise separable upper and lower receivers. The lower receiver houses the trigger mechanism and the upper receiver houses the bolt carrier group. The upper and lower receivers are attached together by a pair of slidable attachment pins. The rear pin can be removed, allowing the upper and lower receivers to pivot relative to one another on the forward pin. The forward pin may also be removed in order to completely separate the upper and lower receivers. Typically, the attachment pins are captured on the lower receiver to prevent loss when the receiver parts are disconnected.
The ability to customize and accessorize the AR15- and AR10-platform has contributed to its overwhelming popularity. The platform is relatively easy to assemble from parts and many users choose to assemble an AR15-platform firearm on their own, allowing them to select exactly which parts, components and features they wish to include. The lower receiver is considered to be the “firearm” and regulated under US law, while the other parts may be purchased and manufactured with relatively broad freedom. The upper and lower receivers may be milled from a forged blank or may be completely milled from a billet of material, usually an aluminum alloy. Published MIL-SPEC standards are typically followed to assure that any standard upper receiver will functionally mate to any standard lower receiver by insertion of the two attachment pins through openings formed in the lower receiver that correspond to openings in attachment lugs on the upper receiver.
Despite following standard specifications, the “fit” between an upper and lower receiver can be imperfect. Some manufacturers intentionally design the fit of their upper and/or lower receiver to have a small gap between the parts when they are assembled in order to allow for manufacturing variances. This gap is considered unsightly by some users, an indication of overly large manufacturing tolerances, or may even result in a lose fit between the upper and lower receiver, allowing movement and rattle to occur. Some manufacturers produce and sell “matched” upper and lower receiver sets, which have been either hand selected for fit or have been drilled to provide an exact fit, at the risk of non-compliance with standard specifications and loss of the ability to universally fit one of the matched pair to any other “standard” receiver.
Although firearms assembled from mill-spec upper and lower receivers have proved to be quite durable, even in combat use, some users are concerned about wear occurring as steel attachment pins are pressed into and removed from the attachment openings in the aluminum upper and lower receivers.